toolchain/gdb: use expat library to be able to parse target machine definitions

Fixes gdbserver debugging with at least mvebu, possibly others

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46298 3c298f89-4303-0410-b956-a3cf2f4a3e73